Great progress has been made in radiotherapy and radiosurgery recently in dosage planning. People are striving to move treatment more and more in the direction of radiosurgery, i.e. working with high radiation dosages applied in a few, and preferably in just a single, radiation treatment to a target volume, so for example to a tumour. Although dosage planning is, as mentioned, relatively successful, the use of high doses administered in a few or a single fraction is often obstructed by the fact that the patient and/or the body section to be irradiated can be positioned only relatively imprecisely. In order to avoid significant damage to healthy tissue, one therefore falls back in most cases on conventional fractionated radiotherapy, in which repeated irradiation with small doses is applied.
In order to improve positioning, one is currently still making do with a very imprecise “manual” method, whereby an x-ray image of a body section of the patient is produced on the linear accelerator. This image is compared with a reference x-ray image previously taken on the simulator (an x-ray device with an identical geometry to the linear accelerator). The doctor carrying out the treatment then compares the x-ray image and the simulator image, for example on a viewing box, thereby determining the positioning error between the actual position of the patient and the desired position using a ruler and then shifting the patient accordingly. At best, a centre-beam cross and/or the contour of the outer field boundary in both images are also available to the doctor as a starting point. The field boundaries may be defined by lead blocks or mobile radiation screens, respectively. Even when comparing with DRRs (“simulator images” virtually determined from a three-dimensional image data set) instead of with actual simulator images, this method does not change.
Disadvantageously, this way of positioning the patient is imprecise, for the following reasons alone:
The images are projective, and therefore not to original scale. (No uniform image scale exists).
The “manual” reading of the required shift is imprecise.
A three-dimensional spatial shift from two-dimensional images and without computer assistance is only possible to a limited extent, and requires a very experienced user.
An iterative method for aligning therapy radiation with a treatment target is known from U.S. Pat. No. 5,901,199, wherein diagnostic computer tomography data are used, with the aid of which a multitude of reconstructed x-ray images, so-called DRRs (Digitally Reconstructed Radiographs), are generated. These DRRs are repeatedly produced and compared with a x-ray image taken at the source, until one is found which shows a sufficient correspondence. With the aid of the data thus obtained, the position of the treatment device and/or of the beam used for treatment is corrected such that the beam hits the treatment target.
A disadvantage of this method is the high computational demands, since such DRRs initially have to be generated at random, and a great many DRRs have to be compared with the actual x-ray image. In particular, an “intelligent” algorithm needs to be found in order to approach the matching DRR for each body section and for each patient in turn in a reasonable period of time.
Furthermore, a method is known in principle for producing x-ray images at the source in a treatment room, in order to integrate the up-to-date information thus gained about the position of the treatment target and its surroundings into the course of the treatment, whereby two securely assembled x-ray sources are regularly used laterally above the patient in radiation treatment, as well as two securely installed image recorders, e.g. let into the floor of the treatment room, with a separate image recorder for each x-ray source. These systems are inflexible and costly in terms of apparatus, and therefore also expensive.